1. Field of the Invention
This invention relates to passivation of group II-VI semiconductor materials and particularly to the passivation of mercury cadmium telluride (HgCdTe).
2. Brief Description of the Prior Art
Passivation of HgCdTe in accordance with the prior art included cleaning of the surface thereof, then passivating with a chemical solution and then depositing a material over the passivated surface. A problem encountered is that some of the passivating materials used are not suitable for an infrared detector, some contribute background noises and some are not mechanically or chemically stable and there are also problems with radiation hardness of the devices. Cadmium telluride (CdTe) has generally been used as the passivating material in the prior art. The CdTe is deposited on the HgCdTe which has a thickness of about 20 to 30 mils and is heated to about 300.degree. C. The mercury then diffuses into the CdTe and the cadmium diffuses into the HgCdTe to provide a graded rather than definite interface. It is believed that this passivation reduces 1/f noise from the HgCdTe surface.
In the fabrication of high quality, low noise photodiodes in HgCdTe, it is necessary to properly passivate the surface of the HgCdTe. It has been shown that CdTe passivated HgCdTe produces low noise, low leakage diodes in thick (&gt;10 mil) HgCdTe when the CdTe is interdiffused by annealing the CdTe/HgCdTe samples at 300.RTM. to 400.degree. C. for up to several hours.
The above noted procedure operates well on the thick HgCdTe samples because the samples do not require mounting to or support by any other structures. They can be annealed as self entities by conventional methods as set forth hereinabove. This forgoes any thermal mismatch with other materials (i.e., no coefficient of thermal expansion mismatches). When thin samples are used which cannot be autonomous there are stresses induced in the HgCdTe which cause dislocations, slip lines, microcracks and fractures in the material. Defects in the HgCdTe severely degrade device performance. This problem is even more severe in the fabrication process used to fabricate FPAs because vertically integrated photodiodes are mounted to silicon wafers by the use of low outgassing epoxys. Since the devices are operated at 77.degree. K, they must be compatible with the process at these temperatures. The interdiffusion anneals can require temperatures at high as about 675.degree. K (about 400.degree. C.). These epoxys are made to be used in limited temperature ranges which do not normally see 600.degree. K excursions. The interdiffused CdTe/HgCdTe is required to produce low noise IRFPA detectors. This problem occurs at about 180.degree. C. However, since a temperature of about 250.degree. to 300.degree. C. is required for the interdiffusion to take place, it follows that the 300.degree. C. passivation temperatures cannot be tolerated in the fabrication of detector system of the prior art as discussed above which utilize an epoxy. The high temperature anneals cause the epoxy to cure and harden. One theory is that the glass transition temperature (Tg) shifts up and the epoxy locks in at higher temperature on cooldown from the anneal. When cooling to device operating temperatures (77.degree. K), the stresses induced due to the coefficient of thermal expansion mismatch are increased even more than normal. These stresses have been shown to cause catastrophic damage to the HgCdTe. According to a second theory, since it is known that the stress induced by thermal mismatch must be greater than the yield stress of the HgCdTe for damage to occur, as the HgCdTe-epoxy-silicon stack is heated, the thermal mismatch between HgCdTe and silicon increases while the yield stress for HgCdTe decreases. This would also be a cause of severe damage of the type noted above. These two factors can cause HgCdTe damage when cooled back down to room temperature.